There are many instances in which it is desirable to obtain informative photographs of objects whose depth of field and distance from the objective lens are comparable and furthermore are not very large compared to the objective lens diameter. Such cases concern near-field imaging which may include both industrial photography and microscopy.
At least three reasons exist why it is often advantageous to first make a hologram of the object and then photograph hologram(s): (1) one can record the "frozen" image of a transient phenomenon in three dimensions and determine later which focal plane(s) and angle(s) should be duplicated photographically; (2) double pulse holography may be employed so that fringes appear on the hologram indicating displacement between pulses; (3) hologram interferometry can be used to form fringes induced by contour or refractive index gradients.
There are, however, two troublesome problems in making photographs from conventional holograms: (1) for near-field images, there is a depth of field problem; i.e., no plane exists in which all of the image structures are in clear focus. Aperturing the camera lens would help in photographing the hologram, but this increases graininess (speckle) in the image, both foreground and background. (2) For large objects, the illuminating laser beam generally must diverge for complete coverage when the hologram is being made. This causes excessive light intensity in the image foregound relative to the image backregion. This undesirable situation is transferred to the photograph.
It is possible to obtain video images directly from the sum of image and reference beam light (in the hologram plane) without ever developing a hologram. The fringes caused by interference between reference and image light, which are normally recorded and developed in the holographic plate, are sensed directly by the video camera. Provided the image is focused in the hologram plane and an appropriate bandpass filter is used in the video electronics, one can see a two-dimensional image on the video monitor screen. Therefore, whatever advantages (depth of field increase, etc.) an apparatus imparts to a hologram can be imparted to the video monitor screen in realtime.
The apparatus of the present invention applies to the two problems outlined above in photographing holograms and also exhibits superresolution benefits found in Doppler spread imaging. The advantages to either photography or video monitoring of using the disclosed system over conventional holographic arrangements are as follows: (1) Greater depth of field, (2) attenuation of excessive foreground intensity caused by diverging object illumination light, (3) resolution improvement in one dimension when the object is seen through a degrading medium.
Examination of three-dimensional objects in a light-scattering medium (e.g., a biological structure imbedded in translucent tissue or electronic circuits in slightly colloidal potting compound) can be improved with the disclosed system. The same Doppler holography which permits greater depth of field also permits improvements in image quality impaired by a light-scattering or distorting medium. (The present invention differs from some previous generalized Doppler spread superresolution systems mainly in that it is specifically arranged for near-field imaging rather than far-field imaging, i.e., a diverging illumination source instead of collimated source; also it is arranged for economy of components and equipped to virtually eliminate shadows normally found in holographic images due to the object illumination.) Since resolution improvement is in one dimension only, successive object rotations in a vertical plane along the system axis (perpendicular to the Doppler-inducing rotation) will result in images with a variety of directions in which resolution is improved. Superposition of transparencies made from photos of such images (or computer-controlled multiplication of video images) can bring out detail normally hidden. Thus, the disclosed system, by being designed to reduce the depth of field problem, also improves resolution.